The present invention relates to a self-discharging centrifugal drum for clarifying and separating suspensions that has an axially displaceable piston valve than opens and closes expulsion openings in the circumference of the drum jacket to allow the removal of centrifuged solids, in which the top of the piston valve is bounded by the solids space and its bottom by a closure compartment into which closure fluid can be pumped, in which the closure fluid is supplied through a channel that is connected to the closure compartment, in which the closure compartment is evacuated by means of a centrifugal valve in the drum jacket, and in which the centrifugal valve is connected to the closure compartment by an evacuation channel that begins at the outside of the closure compartment.
A centrifugal drum of this type is known, for example, from German Pat. No. 2 436 285. This centrifugal drum can be what may be called partially or completely discharged, depending on the suspension and on the centrifuged solids, by means of the axially displaceable piston valve. In this process part or all of the drum packing is expelled through expulsion openings in the circumference of the drum.
Two conditions are necessary to ensure uniform expulsion of the drum packing and especially of the solids centrifuged into the outer zone of the solids compartment. First, the piston valve must open suddenly, leaving a wide opening gap between it and the site where it is sealed off against the drum cover. Second, the piston valve must close suddenly, especially when discharge is only partial. To ensure a uniformly wide opening gap in the drum during both partial and complete discharge, all the closure fluid must always be evacuated from the closure compartment, which must then be recharged with closure fluid at a rate that corresponds to the desired level to which the drum is to be discharged.
Although known centrifugal drums do allow rapid total discharge of the drum packing with a wide piston-valve opening gap, they can not be precisely partially discharged because the closure compartment can not be uniformly and rapidly enough recharged with fresh closure fluid. Closure fluid is introduced from a supply connection into an annular compartment near the axis of the drum jacket and then transported out through channels into the closure compartment, which always takes a relatively long time because the closure fluid, which is introduced under practially no pressure, must be accelerated and brought up to pressure in the centrifugal drum.
For these reasons some self-discharging centrifugal drums are often designed only for partial discharge and others only for complete discharge of the drum packing.
Since, however, there are so many applications for self-discharging centrifugal drums, versions have become known that are supposed to be equally effective for either partial, and uniform, or complete discharge. German Pat. No. 2 048 429 for example discloses a centrifugal drum with a two-part closure compartment between the piston valve and the surrounding part of the drum that is charged with closure fluid before centrifuging commences. Each of the two divisions in the two-part closure compartment is connected to a fluid-activated centrifugal valve that evacuates the closure fluid. Whereas, during partial discharge, the centrifugal valves remove closure fluid from only the inner or the outer division, during complete discharge closure fluid is removed simultaneously from both divisions. In the latter case both centrifugal valves must be opened uniformly, which is, however, very difficult in practice.
For these reasons the self-discharging centrifugal drum known from German Pat. No. 2 704 903 was proposed. This drum also has a two-part closure compartment and two valves, and the channel that evacuates the inner division of the closure compartment opens into the outer division. The centrifugal valves are located in the evacuation channels. The communication between the divisions of the closure compartment causes the centrifugal valve associated with the inner division to open during complete discharge when the outer centrifugal valve, and hence the outer division, open, so that both the outer and the inner divisions will evacuate uniformly and in sequence. This opens the drum rapidly. In partial discharge it is essentially only the outer division that is evacuated. The relatively long channels required to charge the divisions of the closure compartment in this drum, however, make it also close rather sluggishly.